Sealing composition and method

ABSTRACT

A composition of matter and method of temporarily sealing selected intervals of boreholes, said composition comprising a combination of at least one galactomannan gum, a buffering agent, a cross-linking and hydrating agent for said gum, a suspending agent for said gum, and, optionally, degradation agent for said gum and water. The method comprises injecting said composition into said interval in a borehole.

United States Patent [72] Inventor Kenneth H. Nimerick Tulsa, Okla.

[21] Appl. No. 730,578

[22] Filed May 20, 1968 [45] Patented Oct. 26, 1971 [73] Assignee TheDow Chemical Company Midland, Mich.

[54] SEALING COMPOSITION AND METHOD [56] References Cited UNITED STATESPATENTS 3,372,072 5/1968 Chrisp 149/20 3,301,723 1/1967 Chrisp 149/203,096,284 2/1963 Slate 252/855 A OTHER REFERENCES Chem. Abst. Vol. 52:6837, 1958 Copy in Chem. Library Primary Examiner-Lorenzo B. HayesAttorneys-Griswold 8L Burdick, C. Kenneth Bjork and Bruce M. KanuchBACKGROUND Many methods have been employed for the treatment employfibrous, boreholes such as those found in gas wells, oil wells andsimilar boreholes, to seal and plug off selected intervals. Three of themore common methods have been to employ fibrous, flaky or granularplugging agents, cementing the boreholes, and employing various viscousgels as plugging and sealing agents. All of these present methods sufferfrom certain disadvantages. Namely, the use of fibrous materials, e.g.,hay, sponges, etc., has not been entirely successful and in manyinstances an impervious seal is not obtained in vuggs or fissures evenwhen large quantities of material are pumped into the zone. The use ofcement requires an extended period of time to allow the cement to setand to drill through the cement once circulation has been regained.Furthermore, the use of cement can be quite expensive and resultantdamage to producing formations can be extensive. The more recent use ofgels, such as cross-linked polysaccharides, has proved to be a greatimprovement over the former two methods. However, difficulties found inthe use of many of these gels include, instability of the gels with pHchanges, nonuniformity of the gel solutions placed into the borehole,premature setting up of the gels, prolonged setting time, failure of thegel to break down within a certain predetermined period of time, highfluid loss of the unset gels, and weakness of the set gel to extrusioncaused by pressures. Many of these disadvantages have been overcome bythe use of a novel temporarily plugging gel combination consisting of agalactomannan gum treated with a hydrophobing agent, a synthetic waterdispersible polymeric thickening agent, a buffering agent, across-linking and hydration agent, optionally, a degradation agent andwater. The present temporary plugging gel composition demonstratesuniformity, strength, predictable gellation and setting times, goodfluid retaining characteristics, and stability to a wide range of pHvalues. The gel solution may be prepared at the use site employinglocally available fresh water supplies having a wide divergence in pHranges e.g., 3 to 10.

SUMMARY The present invention consists of employing as a temporarysealing and plugging agent in the plugging of selected intervals inboreholes a temporary plugging gel composition comprismg:

a. a hydrophobically treated galactomannan gum;

b. a water-swellable suspending agent;

c. a buffering agent;

d. a cross-linking and hydration agent for said gum;

e. water, and

f. optionally, a degradation agent.

The constituents are admixed together with a sufficient amount of waterto make a readily pumpable mixture. The slightly viscous gel solution isintroduced into the formation to be sealed where it autogeneously gels,i.e. sets up, to give a temporary, fluid-tight pressure-resistant, rigidgel and subsequently disintegrates and ceases to plug.

PREFERRED EMBODIMENTS In the practice of the present invention, sealingand plugging gel compositions gallons of said slurry: combinations ofconstituents based on pounds of constituent per 1,000 gallons ofsolution; unless otherwise noted.

Water Balance to make 1,000

gals of solution Examples of hydrophobically treated galactomannans areguar, tragacanth, Karaya, acacia, tara, locust bean, neutral salts ofalginic acid and the like. The preferred treating agent to render thegum less hydrophilic (referred to as a hydrophobing agent) is aboron-containing compound, e.g., alkali metal borates, such as sodiumperborate, sodium metaborate, and sodium tetraborate. Details of onemethod of treating galactomannan gums with hydrophobing agents aretaught in U.S. Pat. No. 2,868,664. It is preferred to use a sufficientamount of hydrophobing agent in treating the gum so that the pH of anaqueous dispersion of the gum is about neutral or slightly alkaline. Bytreating the gum with a hydrophobing agent, the gum is very slow tohydrate in aqueous solutions having a neutral or alkaline pH but willhydrate very quickly in acidic aqueous solutions. The exact amount ofgum employed for gel stability will depend to some extent on thetemperature to which the set gel will be subjected in the formation tobe sealed. At higher temperature levels, e.g., greater than about F., agreater amount of the gum should be present. For example, above 125 F. agel composition containing guar gum, polyacrylamide and sodiumdichromate at a total concentration of about 500 pounds/960 gallons ofwater has been found to be satisfactory whereas a concentration of onlyabout 300 pounds/980 gallons of water was found practical for use attemperatures ranging from about 60 F. to 125 F.

The suspending agent usually consists of a synthetic waterdispersiblepolymer which hydrates very quickly in aqueous solutions. It is usuallypreferred that the suspending agent consist of one which forms a nearneutral pH in aqueous solutions. One type of polymer found to beespecially effective for this purpose is acrylamide polymers, includingspecifically polyacrylamide in varying degrees of hydrolysis andacrylamide copolymerized with one or more ethylenically unsaturatedmonomers such as acrylic acid, methacrylic acid, vinylsulfonic acid,vinylbenzylsulfonic acid, vinylbenzenesulfonic acid, and alkali andalkaline earth metal salts of such acids as described in U.S. Pat. No.2,831,841, column 3, lines 34 to 45, and in U.S. Pat. No. 2,909,508,Examples 1 to 5. Other materials found effective are cellulose, etc.Preferably the suspending agent is employed in an amount ranging from0.5 to 10 percent by weight of the galactomannan employed. The exactamount of suspending agent will depend on the amount of gum employed,temperature, and other constituents present in the gel solution. Thepresence of the suspending agent forms a slightly viscous solutionthereby providing an even dispersion of the galactomannan gums insolution and prevents fluid loss of the initial solutions until the gumis hydrated and cross-linked.

A buffering agent is employed in order to adjust the pH of gumspretreated with hydrophobing agent so that water having a wide range ofpH values may be employed in preparing the indicated sealingcomposition. This allows for the preparation of the gel solutions at theuse site, employing fresh water supplies or NaCl brines. The bufferingagent should be employed in an amount so as to maintain a pH of near 8.0in the initial solution prior to adding the hydrating and cross-linkingagent. Suitable buffering agents include sodium bicarbonate, boricacid-borax, sodium carbonate-sodium bicarbonate mixture, and the like.It has been found that the present gel solution can be preparedemploying water supplies having pH values ranging from about 3 to about10. At a pH lower than 3 the gellation period is usually too quick, andat a pH higher than 10 the setting time (period for gum to hydrate andgel) is unduly prolonged.

The cross-linking and hydration agent consists of a compound which willlower the pH of the gel solution to a value where the galactomannan willhydrate and set up. It is preferred to employ a compound that induceshydration of the gum and contains a polyvalent metal cation which willcrosslink the hydrated galactomannan gum employed. The crosslinked gelis found to be much stronger than a corresponding uncross-linkedhydrated gum. lt has been found that those polyvalent metal cationswhich form insoluble hydroxides at pH values lower than about 7 are mostbeneficial for crosslinking the galactomannan gums. Polyvalent metalcations include for example, zirconium, chromium, and tin. Cross-linkingand hydration agents most desirable in the practice of the presentinvention consist of polyvalent metal salts of strong acids whichinclude, for example, sodium dichromate, stannic chloride, zirconylchloride, chromium nitrate, and other salts of strong acids whichcontain polyvalent metal cations which form insoluble hydroxides at pHvalues lower than about 7. The indicated salts, upon hydrolysis, lowerthe pH of aqueous solutions thus allowing the gum to hydrate and alsoprovide the polyvalent metal cation for cross-linking the hydrated gum.Other cross-linking agents include a combination of polyvalent metalcompounds which are soluble in aqueous solutions but which do not lowerthe pH of such solutions. Such polyvalent metals are used in combinationwith an acid which has a characteristic time-delayed hydrolysis rate andthus a time-delayed lowering of the pH of the solution. Such polyvalentmetal compounds include, for example, sodium chromate, sodium tungstate,sodium molybate and the like. Suitable acids for use in combination withthe latter compounds include propionic acid, maleic acid, succinic acid,tartartic acid, citric acid and other like acids. The cross-linkingagent in the form of the first defined type may be employed in an amountranging from 0.01 to about percent by weight of the galactomannan gum tobe cross-linked. The exact amount will depend on such factors as gum andbuffer concentration and the temperature of the gel solution. Sodiumdichromate and chromium nitrate have been found to be preferredcrosslinking and hydration agents. They dissolve quickly in the aqueoussolution, have a characteristic time-delayed hydrolysis rate and arecrosslinkers for the galactomannan gums, e.g., guar gum.

A degradation agent is optionally employed to allow the disintegrationof a set gel after a predictable predetermined period of time. Thespecific degradation agent employed will depend on the temperature towhich the set gel is subjected. At temperatures ranging from about 125to 200 F., KBrO and other like materials are used to control degradationof the set gel. At temperatures ranging from about 60 to 140 F., anenzyme may be employed. The selection of a suitable enzyme for aparticular gum can be determined from references well known in the art.The amount of enzyme employed in any particular gel solution as definedherein will depend on the amount of galactomannan gum present, and alsoupon the temperature and pH to which the set gel is to be subjected.Generally, about 0.0 to 2.0 percent by weight of enzyme of the weight ofgum employed is satisfactory.

In practice, the sealing composition is prepared by thoroughly admixingthe galactomannan gum, suspending agent, buffering agent and degradationagent in the necessary amount of water. The cross-linking and hydratingagent is thoroughly admixed with the so-prepared solution. The sealingcomposition is then pumped into the formation to be sealed and the gelwill set up. The set gel will break down within a predetermined periodof time, thus restoring use of the treated zone.

The following examples will facilitate a more complete understandingofthe present invention, but they are not meant to limit the inventionto the specific embodiments incorporated therein.

EXAMPLES l-l8 The effect of the pH of a water supply on setting time(hydration rate) of a sealing composition within the scope of thepresent invention was shown in the following manner. Two formulationswere employed as test medium. The first formulation (noted as blend A)consisted of, as percent by weight, 93 percent guar gum, 5% NaHCO, and 2percent of a commercially available polyacrylamide which demonstrated apH value ranging from about 8 to 10 when 10 pounds were dissolved in1,000 gals. of water. The second formulation (noted as blend I)consisted of percent guar gum, 8% NaHCO, and 2 percent of a commerciallyavailable polyacrylamide having a substantially neutral pH in aqueoussolutions.

The indicated formulations were thoroughly mixed into tap water having apreadjusted pH value. A sufficient amount of blend A or B was employedto make up solutions equivalent to a concentration of 300 pounds or 500pounds of blend per 1,000 gallons of solution. After the blends weremixed with the water, sodium dichromate (about 3.0 percent by weight)was added and the so-formed gel solution was contained in a glassbeaker. The pH of the water was adjusted by the addition of l-lCl orNaOH prior to the addition of the blend. The pH of the water wasdetermined before and after mixing with each blend. The hydration ratewas determined as the period of time required for the viscosity of thegel solution to reach poise as measured by a Brookfield viscosimeter at60 r.p.m. using a No. 3 or No. 4 spindle. The blend concentration ofeach blend, concentration of sodium dichromate (based on pounds/1,000gallons of solution), pH of water before adding the concentrate, afteradding the concentrate, after adding the sodium dichromate, and the timefor the gel solution to reach 100 poise is set forth in the followingTable I.

As indicated in the Table, at a pH value below about 3, Le, Example 16,the gel solution set up immediately prior to adding the sodiumdichromate catalyst. Likewise, at a pH of about 10 or more in the mixingwater a prolonged setting occurred TABLE I Concentration 1bs./

1,000ga1. H20 Solution pH after p11 after Time (min) Sodium mixingpolymer sodium to reach Example Number Blend Blend dichromate waterformulations dichromate 100 poise B 500 15 l 10. 1 B. 3 7. 4 42 B 300 8l 3. 0 8.0 7. 3 8 B 300 8 l 2. 0 Polymvr gelled when nddud to water 1 pHof tap Water adjusted with HCl or NaOH.

EXAMPLES 19-97 As in Examples ll8, a formulation was preparedcorresponding essentially to blend A of those Examples. in the presentexamples, tests were run to determine the plug life (i.e., the time forthe set gel to degrade to reach a viscosity of 50 poise) of the gel.Various concentrations of blend A (based on pounds of blend A per 1,000gallons of 1,000 cross-linking agent (consisting of sodium dichromate)and a degradation agent were employed; and temperatures to which the setgels were subjected are indicated. In each test, blend A was mixed intotap water. Sodium dichromate was added and then either KBrO, or anenzyme (identified as enzyme breaker D from the Stein Hall Company) wasadded as a degradation agent. The gel l i n. wa 211 1112. 2. 1 4:0slassiax .Th viscosity was determined as in Examples 1-18, at variousperiods of time, and the time (in days) noted when the viscosity of thegel was lowered to 50 poise or less. The temperatures employed, blend ofmaterials,and plug life are noted in the following Table ll. As thesetests show, the degradation rate (plug life) can be adjusted withinpredetermined limits at various concentrations of gel solutions byvarying the amount of degradation agent employed. As also is shown, thehigher the temperature to which the set gel is subjected the shorter isits plug life, all other variables remaining constant. Likewise, thehigher the concentration of the gel concentrate the longer the pluglife, all other variables remaining constant.

TABLE II Coucentrntion of Blend A sodium diconcentrachromate, Enzyme ofKBrO:

tiou, lbs lbs/1,000 concentration in Plug 1,000 gals gals. solulhs./1000 gals. life solution tion solution (days) 300 12 None 300 12 2. 0 1300 12 1.5 2 300 12 1. 0 4 300 12 O. 5 7 300 12 0. 25 22 500 20 None 50020 7. 0 2 500 20 5.0 3 500 20 3.0 7 500 20 2. 5 8 500 20 2.0 13 500 201.5 21 500 20 1.0 (0 500 20 0.5 300 None 23 300 10 2.0 1 300 10 1. 5 2300 10 1. 0 3 300 10 0.5 6 300 10 0. 25 9 500 16 None 500 16 7. 0 1 50016 5.0 2 500 16 3. 0 3 600 16 2. 0 5 500 16 1. 0 8 500 16 0. 6 14 500 16...d0. 0. 25 33 300 8 None 9 300 8 1. 0 2 300 8 0. 5 3. 5 300 8 0. 25 5300 8 0. 1 7 500 12 None 500 12 3 1 500 12 2 2 500 12 1. 5 3. 5 500 12 15 500 12 0.5 8 500 12 0. 25 12 500 12 0. 1 17 300 7 None 8 300 7 1. 5 1300 7 1. 0 2 300 7 0.5 3 300 7 0. 25 4 300 7 0. 1 6 500 11 None TABLE II('uniinucd Concentration of Blend A sodium diconcentrachromate, Enzymeof KBrO;

tion, lbs./ lbs/1,000 concentration in Plug Ex. Temp. 1,000 gals. gals.solu lbs..1,000 gals. life No. F.) solution tion solution (days) 68..500 3.0 2 69.. 125 500 2.0 3 70. 125 500 1. 5 5 71-. 125 500 1.0 6 72.125 500 None 73. 125 500 10 4 74. 125 500 7 5 75.. 125 500 5 7 12.5 5003 11 125 500 1 1Q 500 None 500 10 2 150 500 7 3 150 500 5 5 150 500 3 10150 600 1 500 8 None 21 175 500 8 7 1 86. 175 500 8 5 2 87. 175 500 8 34 88. 175 500 8 2 7 89. 175 500 8 1 11 90. 175 500 8 0.5 16 91.... 175500 8 0.25 18 92- 175 500 8 0. 1 20 93-- 175 500 7 None 14 94. 175 500 75 1 95.. 175 500 7 3 2 96. 175 500 7 2 4 97. 175 500 7 1 8 1 Longer than36 days. 1 Longer than 40 days. 1 Longer than 30 days. Longer than 33days. Longer than 21 days.

EXAMPLE 98l 08 This series of examples was used to determine the fluidloss and formation damage of a gel formulation within the scope of thepresent invention. The blend A formulation of Examples 1-18 was used asthe test formulation. As a comparison, a composition containing guar gum(300 pounds/1,000 gallons solution) phthalic anhydride (3.2 pounds/1,000gallons solution) and an enzyme (2 pounds/1,000 gallons of solution) wasemployed. This was a commercially available standard sealing andplugging composition. The fluid loss and formation damage weredetermined using Berea sandstone core plugs. The penneability of thecore plug to air or to 2 percent calcium chloride brine was determined.

The blend A formulation was slurried into tap water. The degradationagent, consisting of an enzyme (enzyme breaker D) was added, followed bythe sodium dichromate. The slurry was agitated with a Palo stirrer, andthen by a mechanical shaker after the slurry was contained in the Baroidpressure test cell. The time elapsed before applying pressure to thecell was determined by the temperature and the approximate time whichwould be required for the slurry to be pumped into a wellbore. When thefluid-loss test was completed, the plug was sealed in the Baroidpressure test cell until it was degraded. The time required fordegradation was obtained from the degradation tests. The core plug wasremoved and the permeability to 2% CaCl brine again determined. Thedifference between the original CaCl brine permeability and the finalCaCl brine permeability was the basis for calculating the percent damagecaused by the plug. The results of these tests are set forth in thefollowing Table 111. As evidenced by the results obtained, the plugcomposition of the present invention retained fluid much better thananother standard commercially employed composition.

TABLE III 2% CaCl;

brine per- Percent meability damage after plug caused 25 degradation byplug ime Cone. Sodium elapsed 2% CaCl: blend A, dichromate, Enzyme,before brine per- Fluid loss in mls. alter lbs./1,000 lbs/1,000lbs./1,000 pressure meability time elapsed (min.) Ex. Temp. Blendcomgal. gal. gal. applied, before test, No. (F. position solutionsolution solution min. md. 1 4 9 Id "1.2m 311m AA XAm mmmmww a ammmm lPermeability to air. 1 N.T.=Not tested.

,000 centipoise alter 25 minutes hydration.

3 Gel purposely designed to have prolonged Setting time. Plug had lessthan 1 Outlet c osed to retain fluid.

slurry pH to between 6.9 and 7.1. After the slurry had obtained aviscosity of at least 1,000 centipoise, it was poured into a 2-inchdiameter by 12-inch long pipe containing 4 inches of 20-40 mesh sand.The pipe caps were equipped with 'A-inch holes. The lower cap openingwas covered with 20-40 mesh screen to retain the sand. The set gel wasextruded with either air or hydraulic pressure. The volume of set gelexaging the gel. The procedure consisted of slurrying the gum intotapwater. A bactericide was then ad truded per minute was recorded atvarious pressures after aging the gel at 77 F.

The results of these tests are set forth in the following Table V l. 5 22. rl k s .m ma. 5 w m C h n 0 t d c d ry to retard bacterialdegradation of the gum. agent was then added to the slurry.

Ammonium hydroxide or acetic acid was used to adjust the TABLE IVExtrusion Rate of the Gel (mls./min.) at various pressures after agingthe gel at 77 F.

24 hours 4 hours Concentration 500 p.s.i.

of metal salt (percent of gum wt.)

Gum cone, lbs./

1,000 gal. Crosslinking agent Ex. No. Type of gum Sodium alglnate...

Various modifications may be made iii the present invention withoutdeparting from the spirit or scope thereof, for it is understood that weare limited only as defined in the appended claims.

What is claimed is:

l. A thickened slurry composition comprising, as pounds per 1,000GALLONS OF SAlD SLURRY:

a. from 150 to 1,500 pounds of a galactomannan gum characterized asbeing hydrophobic in aqueous solutions having a neutral to alkaline pHand being hydrophillic in acidic aqueous solutions and furthercharacterized in that an aqueous dispersion of the gum has a pH rangingfrom neutral to slightly alkaline;

b. from 3 to 60 pounds of a water-swellable acrylamide polymer as asuspending agent for said galactomannan gum said polymer characterizedas hydrating in aqueous solutions having a neutral to alkaline pH, andfurther characterized in that the pH of an aqueous dispersion of saidpolymer is about neutral;

0. a sufficient amount of a buffering agent to provide an initial pH ofabout 8 in said slurry;

d. from 0.5 to 100 pounds of a cross-linking hydration agent said agentcharacterized as hydrolyzing in an aqueous solution having a pH of about8 after a predetermined period of time to acidify said solution andfurther characterized as containing a polyvalent metal ion which formsan insoluble hydroxide at PH values lower than about 7 and cross-linksthe hydrated form of said galactomannan gum; and

e. water in an amount sufficient to make 1,000 gallons of said slurry.

2. The thickened slurry composition as defined in claim 1 and includingup to 20 pounds of a degradation agent which will break the gel formedby a cross-linked hydrated form of said gum after a predetermined periodof time.

3. The slurry composition as defined in claim 1 wherein said:

a. galactomannan gum consists of guar gum;

b. suspending agent consists of an acrylamide polymer having a molecularweight sufficient to thicken aqueous solutions and suspend saidgalactomannan gum; and

c. cross-linking and hydrating agent consists of a salt containin achromium rnetal ion. 4. The s urry composition as defined in claim 3wherein said bufiering agent consists of sodium bicarbonate, saidcrosslinking and hydrating agent consists of a member selected from thegroup consisting of sodium dichromate, chromic nitrate, or chromiumchloride and said acrylamide polymer consists of polyacrylamide having asufiicient molecular weight to thicken said solution and suspend saidguar gum.

5. The composition as defined in claim 2 wherein said com positionconsists of based on 1,000 gallons of solution:

a. said galactomannan gum consists of guar gum about 300 TO 750 pounds;

b. said suspending agent consists of polyacrylamide about 6 to 30pounds; said buffering agent consists of sodium bicarbonate about 24 to60 pounds;

d. said cross-linking and hydrating agent consists of sodium dichromateor chromic nitrate about 2 to 50 pounds;

c. said degradation agents consist of KBrO or an enzyme about 0 to 10pounds; and

f. water in a quantity sufficient to make a total of 1,000 gallons ofsolution.

6. A method of sealing a selective interval of a permeable formationadjacent to a borehole comprising: injecting into said selectiveinterval a thickened slurry composition comprising, based on pounds ofcomponent per 1,000 gallons of slur- 0 a. from 150 to 1,500 pounds of agalactomannan gum characterized as being hydrophobic in aqueoussolutions having a neutral to alkaline pH and being hydrophillic inacidic aqueous solutions and further characterized in that an aqueousdispersion of the gum has a pH ranging from neutral to slightlyalkaline;

b. from 3 to 60 pounds of a water-swellable acrylamide polymer as asuspending agent for said galactomannan gum said polymer characterizedas hydrating in aqueous solutions having a neutral to alkaline pH, andfurther characterized in that the pH of an aqueous dispersion of saidpolymer is about neutral;

. a sufficient amount of a buffering agent to provide an initial pH ofabout 8 in said slurry;

. from 0.5 to pounds of a cross-linking hydration agent 7. The method asdefined in claim 6 wherein said composition includes in addition up toabout 20 pounds of a degradation agent which breaks the gel formed bythe cross-linked galactomannan gum after a predetermined period of time.

35 3? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.615, 1 Dated 26 October 1971 Inventor(s) Kenneth H. Nimerick It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line h, delete "em" line 5 delete "ploy fibrous" and insert ofin line 63 delete "gallons of said slurry: and insert consists of thefollowing Column 5 line Y, delete "of 1 ,OO(]" and insert --ofsolution)--; in line 32, in the 5th heading of Table II change "Enzymeof KBrO to read Enzyme o r KBrO In columns 7 and 8, Table IV, insert thefollowing headings between Ex. No. lhO and 1&1 after the column headed"Concentration of metal salt (percent of gum wt.

100 psi 200 pSi 500 psi 50 psi 100 psi 200 psi 500 psi 50 psi Signed andsealed this 20th day of June 1972.

Attest:

EDWARD I I.FLETGHER, JR ROBERT OTTSCHALK Attesting Officer Commissionerof Patents

2. The thickened slurry composition as defined in claim 1 and includingup to 20 pounds of a degradation agent which will break the gel formedby a cross-linked hydrated form of said gum after a predetermined periodof time.
 3. The slurry composition as defined in claim 1 wherein said:a. galactomannan gum consists of guar gum; b. suspending agent consistsof an acrylamide polymer having a molecular weight sufficient to thickenaqueous solutions and suspend said galactomannan gum; and c.cross-linking and hydrating agent consists of a salt containing achromium metal ion.
 4. The slurry composition as defined in claim 3wherein said buffering agent consists of sodium bicarbonate, saidcross-linking and hydrating agent consists of a member selected from thegroup consisting of sodium dichromate, chromic nitrate, or chromiumchloride and said acrylamide polymer consists of polyacrylamide having asufficient molecular weight to thicken said solution and suspend saidguar gum.
 5. The composition as defined in claim 2 wherein saidcomposition consists of based on 1,000 gallons of solution: a. saidgalactomannan gum consists of guar gum about 300 TO 750 pounds; b. saidsuspending agent consists of polyacrylamide about 6 to 30 pounds; c.said buffering agent consists of sodium bicarbonate about 24 to 60pounds; d. said cross-linking and hydrating agent consists of sodiumdichromate or chromic nitrate about 2 to 50 pounds; e. said degradationagents consist of KBrO3 or an enzyme about 0 to 10 pounds; and f. waterin a quantity sufficient to make a total of 1,000 gallons of solution.6. A method of sealing a selective interval of a permeable formationadjacent to a borehole comprising: injecting into said selectiveinterval a thickened slurry composition comprising, based on pounds ofcomponent per 1,000 gallons of slurry, a. from 150 to 1,500 pounds of agalactomannan gum characterized as being hydrophObic in aqueoussolutions having a neutral to alkaline pH and being hydrophillic inacidic aqueous solutions and further characterized in that an aqueousdispersion of the gum has a pH ranging from neutral to slightlyalkaline; b. from 3 to 60 pounds of a water-swellable acrylamide polymeras a suspending agent for said galactomannan gum said polymercharacterized as hydrating in aqueous solutions having a neutral toalkaline pH, and further characterized in that the pH of an aqueousdispersion of said polymer is about neutral; c. a sufficient amount of abuffering agent to provide an initial pH of about 8 in said slurry; d.from 0.5 to 100 pounds of a cross-linking hydration agent said agentcharacterized as hydrolyzing in an aqueous solution having a pH of about8 after a predetermined period of time to acidify said solution andfurther characterized as containing a polyvalent metal ion which formsan insoluble hydroxide at PH values lower than about 7 and cross-linksthe hydrated form of said galactomannan gum; and e. water in an amountsufficient to make 1,000 gallons of said slurry.
 7. The method asdefined in claim 6 wherein said composition includes in addition up toabout 20 pounds of a degradation agent which breaks the gel formed bythe cross-linked galactomannan gum after a predetermined period of time.